1. Field of the Invention
The present invention relates generally to radar altimeters, and more particularly to frequency modulated continuous wave (FMCW) radar altimeters used for aviation navigation.
2. Description of Related Art
Frequency modulated continuous wave (FMCW) radar altimeters are used by pilots to determine the altitude of an aircraft in flight-critical situations, such as making an instrument landing in low visibility conditions. A FMCW radar altimeter generally comprises a transmitter that transmits a radio signal toward the ground surface, and a receiver that receives the radio signal after it has reflected from the ground surface. The receiver mixes the transmitted radio signal with the received reflected radio signal and thereby generates a difference signal. The receiver uses the frequency of this difference signal to determine the altitude of the aircraft (wherein the frequency is proportional to the altitude). This altitude measurement is then output to a radar altimeter display located within the cockpit of the aircraft.
One inherent problem with the design of a FMCW radar altimeter is that there will be a certain amount of coupling from the transmitter antenna to the receiver antenna. This antenna coupling is particularly problematic at higher altitudes where the magnitude of the antenna coupling signal is significant compared to the magnitude of the received reflected radio signal. As such, the receiver will occasionally lock on to the antenna coupling signal, and then erroneously use the frequency of the antenna coupling signal to determine the altitude of the aircraft. When this happens, the needle of the radar altimeter display drops to approximately 0 feet, resulting in undue pilot concern or even the loss of the pilot's confidence in the radar altimeter.
An attempt to solve this problem has been to utilize a switched filter in the receiver of a FMCW radar altimeter for the purpose of attenuating the antenna coupling signal at higher altitudes. The switched filter is designed to have one frequency response at lower altitudes (which passes both the difference signal and the antenna coupling signal) and another frequency response at higher altitudes (which passes the difference signal and attenuates the antenna coupling signal). Thus, in operation, the receiver will properly lock on to the difference signal both at lower altitudes (where the magnitude of the difference signal is relatively large compared to the magnitude of the antenna coupling signal) and at higher altitudes (where the antenna coupling signal has been attenuated).
There are several disadvantages, however, associated with the use of a switched filter within the receiver of a FMCW radar altimeter. For example, because the characteristics of a switched filter are fixed, various hardware components of the receiver must be changed in order to modify the filter parameters. As such, the switched filter may not be customized on an individual installation basis. Thus, there is a need for a FMCW radar altimeter that does not use a switched filter to attenuate the antenna coupling signal.